Sig Sauer MPX-PSB Update!

So, it’s been about 7 months since my initial review of the Sig Sauer MPX and I feel like there is need for an update.

To begin with, I’ve finally been able to shoot the gun suppressed. I recently built a Form 1 9mm titanium suppressor and tried it out on the MPX, and coming in with incredibly high expectations, I left with some disappointment… Wait… Disappointment? After that review touting praise and worship over the MPX’s utter brilliance and perfection, there is disappointment? How? Why? What happened?

First, I will state that I still like the gun… A lot… but there were certain reasons why I felt so strongly about this gun in the first place. The MPX has a particular design feature that, when it first came out, made this gun stand out from the crowd: A short-stroke piston system. From my experience using AR-15s, short stroke piston systems usually equate to cleaner running, and more importantly, no gas tube spitting propellant gases straight back through the upper receiver, through the gaps around the charging handle, and into your face. WELL, the MPX apparently didn’t get that memo. Just watch the following video:

I know that the awe inspiring bumpfire (which was the first time I had ever achieved a full mag in one burst-ish, mind you) might not show this problem the best, but there is, in fact, a lot of propellant gas blasting me in the face. NO ME GUSTA. In addition, the gun was exceptionally dirty after shooting. While the sludgy carbon fouling just wiped off while cleaning, it was just as dirty as any straight blowback action… That hurts… That hurts a lot.

How is the gun getting dirty? It’s a piston gun, right? Well, after looking closer at the upper receiver of the MPX, you can see that there is a cavernous opening around the gas block and piston that leads into the upper receiver. Much like the MCX, the innards of the MPX’s upper receiver don’t have any kind of barrier between the inside of the gun and the outside world when the action is open. So, when the action cycles during fire, carbon gases are allowed to flow freely into the upper receiver when excess is vented from the piston, which cake the inside with carbon fouling, travel straight to the back of the gun, and then exit the charging handle slot to hit you in the eyeballs. Design problem? Sounds legit.

At this point, I know what you are thinking. “Hey, wait a second, Mr. Smarty Pants… it can’t be the piston causing the gas in the face, because if it was the piston, you’d still get gas in the face when unsuppressed.” I’d respond by saying, “Hey, shut your filthy whore mouth. I don’t appreciate you ruining my bitch fest with your ‘facts’.” But, the unfortunate truth is that you are correct… I am complaining about the wrong thing… It’s not the piston’s fault that I’m getting gas in the face. So tell me, my good man. Why am I still getting gas in the face? Because gas in the face is just an unfortunate side effect of shooting the MPX suppressed.

Let’s look at how a gas-operated firearm works. When a cartridge is fired, propellant gases accelerate the bullet down the barrel. When the bullet travels past the gas port, propellant gases are then tapped from the barrel and cycle the action. The time it takes for the bullet to travel from the gas port to the exit of the muzzle is called dwell time. The cycling of the action begins during the dwell time, but the bolt doesn’t fully unlock until after the bullet has left the barrel, so much of the schmutz that firing a cartridge produces goes out the end of the barrel. It’s not perfect, however, because a tiny puff of left over gas from the chamber always follows the spent casing being extracted into the action of the gun.

When a suppressor is added to the equation, pressure is still in the suppressor when the bolt fully unlocks and the case begins to extract, causing considerably more gas to get blown back into the action via the now open chamber, through the upper receiver, and into your eagerly awaiting eyeball. That’s the gas you get in the face when shooting the MPX. But, if all guns, piston or direct impingement, are governed by this added dwell time principle, why is it so much worse in the MPX than it is with my piston .300 Blackout. I’ve shot over 900 rounds suppressed (subs AND supers) using the Adams Arms XLP piston system and I’ve never gotten gas in the face AT ALL. Since I’m not an engineer, I suppose it will stay a mystery for now…

Shooting the gun without a suppressor, I give the gun an A+

Shooting the gun WITH a suppressor, I give the gun a B. It’s very reliable and accurate gun using subsonic and supersonic ammunition, but I don’t dig the gun ripping it’s sweet ass in my face as I shoot. While the action has always run like a sewing machine, the gun gets filled with carbon fouling something fierce. After a few mag dumps while suppressed, the action gets pretty gritty as well. It didn’t cause any failures, but makes the experience a little tainted.

After the suppressed MPX experience, I see room for definite improvement: Find a way to seal up that charging handle slot. I can’t tell you why the gun is spitting so much gas into my face, but I can tell you that it’s unpleasant. Moral of this story: The gun isn’t really meant for shooting quiet. Keep it loud and let it roar.


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